Low-cost, four-channel cochlear implant

ABSTRACT

A low-cost, four-channel cochlear stimulation system utilizes a completely passive, implantable receiver/electrode array that is inductively coupled to an external wearable processor. The receiver/electrode array is formed in a silicone rubber carrier adapted to be implanted in a deaf patient. At one end of the receiver/electrode array, positioned subcutaneously near the surface of skin above the ear, four receiving coils are arranged in an appropriate pattern. Such receiving coils are held within an hermetically-sealed titanium case. At the other end of the receiver/electrode array, which may be pre-formed in a spiral to match the basal turn of the cochlea, and which is inserted in the cochlea, four ball electrodes are spaced apart along an inner radius of the spiral. Each electrode is electrically connected to a respective receiving coil. Each receiving coil is also electrically connected to a reference electrode typically located near the receiver-coil end of the array. The wearable processor senses audible sounds, converts the sensed sounds to corresponding electrical signals, and splits the electrical signals into four frequency bands or channels. A speech processing strategy applies the processed signals of each channel to each of four external coils, as a series of biphasic current pulses. The four external coils are aligned, using a suitable headpiece, with corresponding coils of the receiver/electrode array, thereby inductively coupling the biphasic current to a respective electrode of the implanted electrode array.

This application is a continuation-in-part of U.S. application Ser. No. 08/328,260, filed Oct. 24, 1994, now U.S. Pat. No. 5,549,658, issued Aug. 27, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to low-cost cochlear implants, and more particularly to a low-cost cochlear implant that artificially stimulates the cochlea as a function of sensed audio signals to improve hearing of the hearing impaired.

Cochlear implant devices and systems are in worldwide use to restore useful auditory sensations to deaf patients. Such devices and systems take numerous forms, as seen, e.g., in U.S. Pat. Nos. 3,751,605; 4,357,497; 4,400,590; 4,408,608; 4,532,930; 4,9118,745; or 5,095,904, all of which typically include an external (or wearable) processor and an implanted stimulator. The external processor includes circuitry for sensing audio sounds, processing the sensed sounds in accordance with an appropriate speech processing strategy, and transmitting a signal representative of the sensed and processed sounds to the implanted stimulator. The implanted stimulator includes appropriate electronic circuitry, usually powered from electromagnetic energy contained within the signal transmitted from the external processor, that receives the transmitted signal, extracts operating power therefrom, decodes it, and applies an appropriate stimulation signal to one or more electrodes of an implanted cochlear electrode array. The electrodes of the array are implanted within the cochlea, and when energized cause an electrical current (stimulation pulses) to flow, in accordance with a pattern and amplitude as determined by the speech processing strategy, and stimulate the neurons of the auditory nerve. Such stimulation imparts to a patient the sensation of hearing. Performance of some patients who use such cochlear stimulation systems is good enough to allow conversation over the telephone.

Recent advances in speech processing strategies have demonstrated that the level of speech recognition in all implant patients may be significantly improved. See, e.g., Wilson et al., "New levels of speech recognition with cochlear implants," Nature, Vol. 352, pp. 236-238 (1991). Unfortunately, the high cost of present commercially-available cochlear implants makes them unaffordable for many patients. What is needed, therefore, is a low-cost cochlear implant device that would allow the recent advances in speech processing strategies to be more widely used and available to deaf patients than has heretofore been possible.

A low-cost cochlear device must not achieve its low cost at the expense of patient safety. In any implanted electronic device several potential hazards exist for the patient. For example, all materials must be fully biocompatible. If toxic internal components are necessary, they must be encapsulated in an impenetrable casing (hermetically sealed) to protect surrounding tissue. All manufacturing and construction techniques must be performed in a suitable clean room or laminar flow hood to reduce the possibility of dust or other airborne contaminants from entering the fabrication materials. Further, all components used in the overall design must assure reliable operation over a long period of time, preferably for the lifetime of the patient. As the number of components within the implanted device decrease, these safety and reliability concerns can generally be kept at manageable levels. Hence, there is a need for an implantable cochlear stimulator having a low parts count.

The first goal of any auditory device, of course, is to provide the user with a reasonable level of auditory function. Like most evolving technologies, what is "reasonable" has changed in recent years as the performance of cochlear implant devices has improved. If reasonable speech recognition is defined as 20% correct or better using only sound sensed through the implant for everyday sentences or their equivalent, then a multi-channel device is necessary.

For purposes herein, "multichannel" means that stimulation may be applied to the cochlea at several physically-separated sites, each of which is selectively and independently controlled by respective electronic circuits, or "channels", in accordance with the desired speech processing strategy. It is generally agreed that having multiple channels provides improved auditory response. However, the optimal number of channels to use, particularly in view of the increased cost and complexity of the device, remains an unanswered question.

Multichannel devices should not be confused with single channel cochlear implant devices, such as is shown in U.S. Pat. No. 3,751,605, where the information contained in a single information channel is transmitted and applied to an intra-cochlear electrode. While single channel devices can be made much simpler and less costly than a multichannel device, they only provide some sound awareness and limited speech discrimination. As such, they are primarily useful only as an aid to lipreading. In contrast, present day multichannel devices provide open-set speech recognition in the 30-60% range of most patients, with some patients being able to converse over the telephone with strangers.

Likewise, multichannel devices should not be confused with simple multielectrode schemes where effectively a single channel of auditory information is applied, on a shared basis, to multiple electrodes, such as is described in U.S. Pat. No. 3,449,768. Although the '768 patent refers to its separate electrode pairs as separate "channels," they are clearly not separate channels as that term is used today to describe cochlear stimulating devices. Multi-electrode schemes driven by a single auditory channel of information, such as that depicted in the '768 patent, have long been abandoned as ineffective at producing acceptable speech recognition.

Although some patients with multichannel devices do not perform well, such should not be viewed as an indication of the failure of multichannel devices. Rather, such failure probably reflects the fact that some patients with poor nerve survival, electrode placement, or device adjustment cannot recognize speech even with multichannel implants. Thus, while there is a wide variation in performance for all patients, over 20 years experience indicates that a multichannel implant is necessary to achieve reasonable speech recognition.

Disadvantageously, the use of multiple channels significantly increases the cost and complexity of the device, particularly the implant device. Yet, as indicated above, a multichannel device is much preferred over a single channel device in order to significantly improve the level of auditory function provided by the device. Hence, it is apparent that there is a need in the art for a multichannel device that avoids the complexity and expense of existing multichannel devices, is inexpensive to manufacture and maintain, and is safe to implant and use.

A further contributor to the high cost of cochlear implant devices is hermetic sealing. Any electronic package implanted into the human body must be hermetically sealed to protect the delicate electronic components from the harsh body fluids, and to protect the patient from exposure to potentially toxic chemicals contained in the electronic components. Early cochlear implant devices were encapsulated in silicone rubber, epoxy resin and/or bone wax. These devices lasted for many months (and in some cases years), but all failed eventually because water vapor penetrated through the silicone rubber and epoxy, causing corrosion and short-circuiting of the passive components, and/or failure of the active components.

Hermetic sealing, unfortunately, is a complex technology that is still undergoing significant development efforts in related implantable industries, e.g., the pacemaker industry. Titanium or ceramic capsules are sealed with electron beam welding or laser welding or brazing. The electrical leads coming out of the capsule present a particularly difficult problem for the seal. Each lead must be fully hermetically sealed and insulated from the capsule. This is typically accomplished by conductive pins sealed to ceramic collars which are then sealed to the capsule. Electrode wires are then connected to the pins. Hermetic sealing is one of the most costly and difficult processes used in construction of modern cochlear implants. For a truly low-cost cochlear implant device to be made, a low-cost hermetic seal is needed, or the implant device itself must be able to function safely for long periods of time without the need for an hermetic seal.

The present invention advantageously addresses the above and other needs.

SUMMARY OF THE INVENTION

The present invention provides a low-cost multi-channel, cochlear implant system that includes an external portion and an implantable portion. The external portion includes a wearable processor connected to a headpiece via a flexible connecting cable in conventional manner. A microphone senses audible sound signals and converts them to electrical signals. The electrical signals, after preliminary processing, are split into one of multiple frequency bands or channels. The signal in each frequency band is further processed in order to generate a signal that preserves the loudness relations within and between the speech sounds of that band. The signal thus processed is then pulse modulated using a continuous interleaved sampling (CIS) strategy. The modulated processed signal is then applied, via the flexible connecting cable, to respective coils for each band or channel located in the headpiece.

The implantable portion of the cochlear implant system includes a passive, transcutaneous implantable receiver integrally connected to a cochlear electrode array. Electrically, the implantable receiver includes only a receiving coil for each channel, which coil has at least one side electrically connected to a respective electrode of the electrode array. Typically, the other side of the coil is connected to a common reference electrode, thereby providing monopolar stimulation between each electrode and the common reference electrode. In other embodiments, the other side of the coil may be connected to another electrode of the electrode array, thereby providing bipolar stimulation between the two sides of the coil. In a preferred four-channel configuration, the implantable receiver/electrode array simply comprises four implantable coils, each having one end thereof electrically connected to a respective electrode of the cochlear array, and the other end electrically connected to the common reference electrode.

In accordance with one embodiment of the invention, there is no need for hermetic sealing of the implantable portion. That is, because there are no active components in the implantable portion, nor even any passive components except for the wire from which the coils and electrodes are made, and because the wire may be selected from biocompatible materials so that it is thus of medical grade, the implantable portion need not be hermetically sealed. The wire may be multistranded for flexibility and reliability, but such is not necessary. The wire is typically insulated with a suitable biocompatible insulating material, such as Teflon. The electrode ends of the wire may be made as balls that are formed by flaming the end of the wire. In order to reduce cost, the coil part of the implantable portion may be made from one type of wire, e.g., copper, and the lead/electrode part of the implantable portion may be made from another type of wire, e.g., Pt-Ir, that is welded to the coil wire. Alternatively, both the coil and the lead/electrode part may be made from one continuous type of wire. The balls, wire and coils are mounted in a carrier of silicone rubber (Silastic) to ensure reliable contact spacing and to impart surgical handling properties for insertion. The large surface area on the balls advantageously produces a low electrode impedance. Such low impedance, in turn, reduces the voltage and power that must be coupled across the coil pairs.

In accordance with an alternative embodiment of the invention, the implantable portion, i.e., coils, wires and electrodes, may be made using less expensive, but non-biocompatible, materials that are hermetically sealed using low-cost hermetic sealing technologies.

The cochlear electrode array is inserted into the cochlea using a special insertion tool of the type described in U.S. Pat. No. 4,819,647. The electrode contacts are cast into the silicone rubber carrier in a pre-formed shape that matches the basal turn of the cochlea. The electrode array typically has a mechanical memory for the coiled shape. The electrode carrier is straightened prior to insertion, and as the carrier is inserted it resumes its coiled shape, forcing the electrode contacts to hug the inner radius of the basal turn, along the modiolar wall of the cochlea.

Not all embodiments of the invention require the carrier to assume a spiral shape. In order to reduce costs, which is an important aspect of the present invention, the spiral design of the array 12 may be replaced with a less-expensive "straight" design. Such straight design still employs a flexible carrier, e.g., silicone rubber, that would allow the distal end 34 of the array 12 to be inserted into the cochlea. Once inserted into the cochlea, such straight array would of course be held in a spiral shape by the cochlea. However, the individual electrodes would not necessarily hug the inner radius of the basal turn along the modiolar wall of the cochlea, as they would if a spiral shaped electrode with mechanical memory were employed.

The implanted coils are held by the carrier of silicone rubber in a particular pattern, or spaced relationship. Such coils are also implanted near the surface of the skin, above or near the ear. One or more permanent magnets, or magnetic pieces, may be centered within or near one or more of the coils. Such permanent magnets or pieces serve as an alignment guide for corresponding coils of the external headpiece. That is, one or more permanent magnets may also be included in the external headpiece such that when the headpiece magnets are aligned with the implantable magnets or magnetic pieces, respective coils in the headpiece are also aligned and held in place with corresponding ones of the implanted coils.

In use, the external processor generates electrical signals representative of the audible sounds sensed by the microphone. Such signals are processed, in accordance with the CIS strategy, resulting in a sequential series of biphasic stimulation pulses being applied to coils held in the headpiece as a function of the auditory information of each channel. The headpiece coils, when properly aligned, are inductively coupled with the implanted coils. Hence, the biphasic stimulation pulses applied to the headpiece coils are inductively coupled to the implanted coils, causing a small, but perceptible, biphasic electrical current to flow between the electrode of the coil and the reference electrode (monopolar stimulation), or between the two electrodes of the coil (bipolar stimulation). Such stimulation activates appropriate neurons of the auditory nerve, thereby imparting to the patient the sensation of hearing.

It is a feature of the invention to provide an inexpensive cochlear stimulation system that provides the best combination of reliability, low cost and robust medical properties for widespread application.

It is another feature of the invention, in accordance with one embodiment thereof, to provide such a cochlear stimulation system wherein the implantable portion thereof need not be hermetically sealed, is made from biocompatible materials, and has a usable life that is at least as long as the expected life of the patient within whom it is implanted, thereby requiring only one surgical implantation.

It is a further feature of the invention, in accordance with another embodiment thereof, to provide such a cochlear stimulation system wherein the implantable portion thereof is made from readily available (and hence less expensive) materials which are hermetically sealed in a receiver module.

It is an additional feature of the invention to provide an inexpensive cochlear stimulation system that utilizes the frequency content of brief bi-phasic pulses generated in multiple channels of a CIS processing strategy to inductively couple such pulses through the skin of a patient to an implanted coil with reasonable efficiency. Such pulses are then used directly as a stimulating signal at electrodes connected to the implanted coil without the need for intervening implanted electrical or electronic circuitry.

It is yet a further feature of the invention to provide a low cost, reliable, cochlear stimulation system that has widespread application in underdeveloped areas, requiring minimal medical supervision and attention once implanted.

It is still another feature of the invention to provide a low-cost, four-channel cochlear stimulation system that utilizes a completely passive, implantable electrode array having four electrodes, four receiving coils, and a reference electrode, carried in a carrier of silicon rubber, one end of which may be pre-formed in a spiral to match the basal turn of the cochlea, with the four electrodes being spaced apart along the inner radius of the spiral.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 is a functional block diagram of a low-cost, four channel, cochlear stimulation system made in accordance with the present invention;

FIG. 2 shows a side view of a passive implantable cochlear electrode/coil assembly made in accordance with the present invention, with the electrode end of the assembly being formed in the general spiral shape of the basal turn of the cochlea, and with the coil end of the assembly maintaining the coils in a fixed spaced relationship relative to each other;

FIG. 3 illustrates a coil included within the electrode/coil assembly of FIG. 1;

FIG. 4 depicts the positioning of the headpiece assembly on a patient over an area where the coil end of the implantable electrode/coil assembly has been implanted;

FIG. 5 is a block diagram of the external processor and headpiece portions of the cochlear stimulation system of FIG. 1;

FIG. 6 is a timing waveform diagram that illustrates the concept of splitting the incoming speech signal into four bands, and sampling the signals of each band in an interleaved (non-simultaneous) fashion;

FIG. 7 depicts the concept of continuous interleave sampling (CIS) as a preferred type of speech processing strategy for use with the present invention;

FIG. 8 illustrates the basic approach used to develop a model of an hermetically-sealed four-coil interface;

FIGS. 9A, 9B and 9C depict the frequency response characteristics of the coil interface model;

FIGS. 10A and 10B define the dimension variables, "A", "b" and "c" used in making the coils to be used within a coil-interface design;

FIG. 11A shows a planar view of a four-coil hermetically-sealed case which comprises part of the implantable portion of the invention;

FIG. 11B shows an edge view of the four-coil hermetically-sealed case of FIG. 10A;

FIG. 12 is a graph that compares the frequency response of the model interface with prototype interfaces with and without a titanium case, and illustrates that the case does not significantly degrade the transmission efficiency at the frequency of interest;

FIG. 13 shows time vs. voltage waveform diagrams illustrating an input biphasic pulse train applied at the input of a transmitting coil (top waveform) and an output biphasic pulse train sensed at a receiving coil hermetically sealed within a titanium case (bottom waveform);

FIG. 14 graphically illustrates the measured cross-talk between coils of different channels of the hermetically-sealed four-coil interface of FIG. 11A;

FIG. 15A depicts a preferred arrangement of magnets within the hermetically-sealed four-coil interface (implantable receiver) of FIG. 11A, which magnets are used to assure a proper alignment with the corresponding coils of an external headpiece (transmitter); and

FIG. 15B shows the preferred arrangement of the magnets and coils in an external headpiece (transmitter) adapted for use with the implantable receiver of FIG. 15A.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

The goal of the present invention is to provide a low-cost cochlear implant device that safely provides a reasonable level of auditory function. A threshold determination for such a device is the number of channels that the device should include.

As discussed previously, as the number of channels decreases, the ability of the system to provide a reasonable level of auditory function decreases. Yet, as the number of channels increases, the size, complexity and expense of the device increases. It does not appear, at least the data is not conclusive, that a corresponding increase in auditory function is achieved as the number of electrodes and the number of channels increases to a higher number, e.g., greater than four processing channels and more than eight electrodes. That is, the test data from patients with all multichannel cochlear implants are similar, even though some devices have six electrodes and four processing channels and others have 22 electrodes and 5 processing channels. While the reasons for the similarity are not fully understood, experiments show that reasonable performance is achieved with as few as four channels. That similar performance is achieved regardless of the number of electrodes and channels used appears to demonstrate the processing power of the human brain responding to aberrant patterns of neural activity originating in the cochlea.

All existing implant processors have inherent limitations in terms of the patterns of nerve activity that they can represent. The similarity of performance achieved with such processors thus probably represents the pattern recognition of the central auditory system when confronted with equally unnatural patterns of peripheral nerve activity. It has been shown, using an acoustical model, that four channels should be adequate to provide adequate or functional levels of speech recognition if all other conditions are optimal. See, Shannon et al., "Mathematical and acoustic models of neural activation patterns in cochlear implants," Assoc. Res. Otolaryngol, p. 143 (1994).

Thus, the present invention contemplates that four channels should be adequate for patients with good nerve survival and an electrode that is well-placed within the scala tympani. However, four channels, may not be adequate for patients with poor nerve survival and/or poor electrode placement. Hence, with poor nerve survival and/or poor electrode placement, it is possible that even higher numbers of electrodes and processing channels would be insufficient to provide such patients with good word recognition without lipreading. While more than four processing channels should theoretically provide more spectral information, and thus better performance, the physical size and cost of design complexity, manufacture, and maintenance increases substantially with the number of channels.

For purposes of the present invention, four channels is the preferred number of channels because it represents a good compromise between the cost and acceptable speech recognition performance in most patients. It is to be understood, however, that less than four channels, e.g., three channels, or more than four channels, e.g., five, six, seven or eight channels, may be used in some situations and with some patients.

Referring to FIG. 1, a functional block diagram of a low-cost, four channel, cochlear stimulation system 10 made in accordance with the present invention is shown. The system 10 includes a passive, implantable receiver/electrode array 12, and an external processing unit 14. The array 12 is made for subcutaneous implantation, below the skin 16 of the patient. The array 12 may be hermetically sealed or non-hermetically sealed. The external processing unit 14 is made to be worn or carried externally (not implanted). The external unit includes, as explained more fully below, a wearable processor 18, powered by a suitable battery 20, and a headpiece 22.

The implantable receiver/electrode array 12 forms a key element of the low-cost stimulation system 10 of the present invention not because of what it contains, but because of what it does not contain. Unlike multichannel implantable cochlear stimulators of the prior art, which have always included relatively sophisticated electronic circuitry for receiving, processing and generating the stimulation signals that are used, the receiver/electrode array 12 of the present invention includes only wires, formed as coils or electrodes, a suitable carrier to hold and position the wires, possibly some bobbins on which the wire can be wound to form coils (although such bobbins are not always needed), and appropriate alignment elements to help align the external headpiece 22 therewith. All such elements are completely passive, may be biocompatible, and may thus be used with or without hermetic sealing. Hence, it is seen that the cost of the array 12 may be significantly reduced over that of prior cochlear implant devices.

More particularly, as seen electrically in FIG. 1, and mechanically in FIG. 2, the implantable receiver/electrode array 12 includes an elongate silicone rubber carrier 26 in which is embedded a plurality of coil assemblies 28 at a proximal end 30 of the carrier and a corresponding plurality of electrodes 32 at a distal end 34 of the carrier. The distal end 34 of the carrier may be formed in the shape of a spiral to match the basal turn of a human cochlea. The plurality of electrodes 32 are spaced apart along an inner radius of the spiral.

The proximal end 30 of the carrier 26 holds the plurality of coil assemblies 28 in a predetermined pattern. In accordance with one embodiment, each coil assembly 28 is made from nominally 800 turns of 40 gage (AWG) insulated Pt/Ir (90/10) wire, or other suitable wire, wound on a disc-shaped Teflon bobbin 36. The bobbin 36 is shaped to form the resulting coils as pancake coils, having an approximate diameter of 1.2 cm, and a thickness of 0.6 cm. The Pt/Ir wire (or other wire) is used for the coil because of its biocompatibility.

Still referring to FIGS. 1 and 2, but with reference also to FIG. 3, a first end 38 of the 40 gage Pt/Ir biocompatible wire of each coil assembly 28 is bonded to a second biocompatible wire 40. The second wire may also be made from Pt/Ir (90/10), or may be made from another suitable Biocompatible material, as discussed below. The second wire 40 may be a braided wire with each braid having a nominal diameter of 2 mils. The second wire 40 extends the length of the elongate carrier 26 inside of the carrier to a respective electrode 32 at the distal end 34 of the carrier.

A second end 42 of the biocompatible wire of each coil assembly 28 is similarly bonded to a third biocompatible wire 44. This third wire 44 is also preferably a braided wire made from Pt/Ir (90/10), but may also be made from another suitable biocompatible material. Each braid has a nominal diameter of 2 mils. Typically, it extends less than half of the length of the elongate carrier 26 to a common electrode 46, exposed at the surface of the carrier 26.

It should be noted that the second wire 40 and the third wire 44 need not be separate wires from the wire of the coil assembly 28. That is, one end of the wire used to form the coil assembly may pass through the body portion of the carrier 26 and end at the electrode 32, with a ball electrode, as described below; with the other end of the wire used to form the coil assembly passing through the carrier 26 to the reference electrode 46. Such single-wire construction avoids a joint between the coil wire and the second wire 40 and the third wire 44. In practice, however, the coil wire will typically be a much smaller wire (smaller diameter) than the second wire 40 or the third wire 44, thereby allowing the coil assembly to have a large number of turns and still maintain a reasonably small physical size. Further, there are some cost benefits, described below, to having the coil wire be one type of relatively inexpensive wire, and the electrode ends to be of another type of wire.

Each electrode 32 at the distal end 34 of the carrier 26 comprises a ball of approximately 0.5 mm diameter. Such ball may be formed by simply flaming the distal end of the second biocompatible wire 40, and positioning the ball within the carrier so that a portion thereof is exposed through the surface of the carrier. The ball electrodes are positioned within the silicone rubber (Silastic) carrier 26 to ensure reliable contact spacing and to facilitate surgical handling properties for insertion into the cochlea. The large surface area of the balls advantageously produces a low electrode impedance, and thus a low compliance voltage is all that is required to drive a charge (current flow) across the electrode/fluid interface.

The reference or common electrode 46 may be made by simply flaming the ends of the wires 44 to form balls, and then positioning the balls of each wire in the same general area on the surface of the carrier 12. Alternatively, the ends of all four wires may be welded together, e.g., flamed at the same time, effectively welding all four ends together in one large ball. Such large ball may then be formed into a desired shape, e.g., flattened, if needed. When implanted, it is preferred that the common electrode 46 be in electrical contact with the temporalis (a fan-shaped muscle situated at the side of the head).

The bonding between the 40 gage wire of the coil assemblies, and the braided wires 40 and 44, is preferably achieved by welding. The weld point between the 40 gage wire and the 2 mil braided leads is preferably reinsulated and strain relieved. Such can be accomplished by the silicone rubber carrier 26 in which the wires and weld point are embedded.

As indicated above, the same type of metal may be used for the coil as well as the second wire 40 (forming the stimulating electrode 32) and the third wire 44 (forming the reference electrode 46). When the same metal is used for the electrodes, wires, and coils, there is no battery effect caused by the use of different metals, reducing the possibility of corrosion of the wires. When the same metal is not used for the electrodes, wires and coils, there may be a battery effect. However, such battery effect may be balanced by having two identical junctions where the coil wire is joined to the second wire 40 and the third wire 44. That is, if the coil wire is stainless steel (which is much less expensive that Pt/Ir), and the second and third wires are Pt/Ir, then the junction of the second wire 40 to the coil (from Pt/Ir to stainless steel) may serve as the positive battery terminal, and the junction of the coil with the third wire 44 (from stainless steel to Pt/Ir) may serve as the negative battery terminal. Other biocompatible metals (or even non-biocompatible metals that are appropriately hermetically sealed and/or encased in silicone rubber, Teflon, or other biocompatible insulating materials, and distant from the exposed electrode end of the array, where body fluids will eventually invade) may also be used within the invention. For example, copper wire may be used for the implanted coils, and Pt/Ir wire may then be used for the electrodes. As explained more fully below in connection with FIGS. 10A et seq., the use of copper wire for the coils is relatively inexpensive, thereby helping to keep the overall cost of the implanted receiver/electrode array 12 low. Gold wire (or a gold-alloy wire) may also be used within the array 12, as gold is usually less expensive than platinum. Any metal that provides the desired electrical properties and can be obtained at a relatively modest cost may thus be used within the array.

It is noted that the carrier 12 may be made from other materials, other than silicon rubber, as such materials are developed and/or made available.

The receiver/electrode array 12 of FIGS. 1 and 2 provides monopolar stimulation, i.e., the current flow occurs between one of the electrodes 32 and the reference or common electrode 40. Advantageously, a well-positioned monopolar electrode 32 produces selective stimulation equal to that achieved using a bipolar configuration, but with a lower current and voltage. To achieve the desired selectivity, the monopolar electrode 32 must be positioned along the modiolar way of the scala tympani, adjacent to the spiral ganglion. To reliably obtain such placement of the electrodes 32, they are (as indicated above) cast into the silicone rubber carrier 26, which carrier may be pre-formed into the shape of the basal turn or spiral of the cochlea. The electrodes 32 are then positioned and equally spaced along the inner radius of the spiral, starting with the first electrode at or near the distal tip and proceeding (as depicted in FIG. 2) with the last electrode being at about 200-220 degrees counter-clockwise from the first (relative to the center of the spiral).

Alternatively, as mentioned previously, ball electrodes may be placed on both ends of the coil wire, with such electrodes then being positioned at the distal end 34 of the carrier to form a bipolar cochlear electrode array.

In one embodiment of the invention, the distal end 34 of the array 12 has a mechanical memory to impart the coiled (or spiral) shape. The array is straightened prior to insertion in the cochlea. As it is inserted, it resumes its coiled shaped, forcing the electrodes 32 to hug the inner radius of the basal turn, along the modiolar wall.

In order to reduce costs, other embodiments of the invention replace the mechanical-memory-shaped spiral design of the array 12 with a less-expensive "straight" design. Such straight design employs a flexible carrier, e.g., silicone rubber, that allows the distal end 34 of the array 12 to be inserted into the cochlea, in conventional manner. Once inserted into the cochlea, such straight array assumes the spiral shape of the cochlea. However, the individual electrodes may not necessarily hug the inner radius of the basal turn along the modiolar wall of the cochlea, as they generally would if a spiral shaped electrode with mechanical memory were employed. Preliminary data suggests that at least some implant patients are able to achieve a high level of speech recognition when the CIS strategy is employed even without a spiral shaped electrode. Hence, based on existing data, the mechanical-memory-spiral-shaped electrode array becomes an option that is not needed for all patients.

A final element included in the implantable receiver/electrode array 12 is one or more, preferably two, alignment elements or pieces 50. Such alignment elements 50, in the preferred embodiment, comprise a permanent magnet, such as a ceramic magnet, e.g., a Zirbidium-Cobalt, rare-earth magnet. A preferred arrangement of such magnets, for both the receiver and headpiece, is illustrated below in connection with FIGS. 15A and 15B. Alternatively, a ferromagnetic plate may be used that is attracted to a permanent magnet 52 within the headpiece 22 (FIG. 1). Other types of alignment elements, other than magnetic, may also be used, such as optical or electromagnetic alignment devices and methods, or mechanical appliances worn on the head.

When the spiral-shaped electrode array 12 is used, the method of making it so that it assumes the desired spiral shape may be similar to that described in U.S. Pat. Nos. 4,686,765 and 4,819,647, incorporated herein by reference. Further, an insertion tool has been designed to hold the straightened electrode carrier in preparation for insertion. Such an insertion tool is fully described in U.S. Pat. 5,443,494, incorporated herein by reference. To facilitate such insertion, and use of the insertion tool, a right angle 48 (FIG. 2) is offset in the silicone rubber carrier 26 at the point of full insertion. The distal end is straightened as it is retracted into a tube of the insertion tool. The tip of the tool is placed on the round window cavity of the ear, or in a cochlear fenestration. Depressing a plunger of the tool inserts the electrode array (distal end 32) into the cochlea reliably and reproducibly, with the offset angle 48 serving as a mechanical stop for the insertion. Use of the tool thus simplifies the electrode array insertion process, and reduces inter-surgeon variability in the insertion position, angle and potential insertion trauma.

The external processing unit 14 includes, as briefly indicated above, a wearable processor (WP) 18 and a headpiece 22, shown in both FIG. 1 and FIG. 4. The headpiece 22 is electrically connected to the WP 18 via a connecting cable 58. The function of the external processing unit 14 is to sense speech signals, or other audible signals, convert such signals to appropriately processed electrical signals, and couple the electrical signals into appropriate channels of the implantable array 12 where they function as an electrical stimulus within the cochlea, stimulating appropriate neurons of the auditory nerve, and thereby imparting the sensation of hearing to the user.

The WP 18 senses speech signals using a microphone 56, which may be conventional. The microphone 56 converts the speech signals to an electrical signal, which electrical signal is processed as described below, and then applied to one of four external coils 54 included within the headpiece 22. When proper alignment exists between the headpiece 22 and the implantable array 12, each of the four external coils 54 is inductively coupled to a corresponding one of the implantable coil assemblies 28. Thus, an electrical pulsed signal appearing at a given one of the external coils 54 is inductively coupled to the corresponding implanted coil assembly 28. Such pulsed signal thus generates a small voltage between the common electrode 46, connected to one side of the implanted coil assembly 28 via the wire 44, and the electrode 32, connected to the other side of the implanted coil assembly 28 via the wire 40. Such voltage causes an electrical current to flow through the conductive media (body tissue) that lies between these two electrodes, thereby producing the desired electrical stimulus.

To achieve the desired alignment between the headpiece 22 and the implantable array 12, the headpiece 22 includes at least one, and preferably two, alignment elements 52. As indicated above, such alignment elements 52, in the preferred embodiment, comprise Zirbidium-Cobalt, or other rare-earth, ceramic permanent magnets that magnetically align with similar magnets or magnetic plates within the implantable array 12, e.g., using the configuration shown below in FIGS. 15A and 15B. The strength of the magnetic attraction can be adjusted by controlling the size and position of the external magnets 52. Too much force can interfere with blood flow and produce flap necrosis, while too little force may result in the headpiece falling off under normal activities.

The coupling of the signals from the external coil 52 to the implanted coil assembly 28 is one of the most critical elements of the design of the cochlear stimulation system 10. Such coupling is what dramatically increases the cost in most prior art systems, and in which acceptable comprises must be made in order to maintain the cost at acceptable levels. The present invention achieves the necessary coupling through the use of the four coil assemblies 28 in the implantable electrode array 12. Such coils are untuned passive coils, and as such, will transmit a brief biphasic pulse with adequate fidelity. Hermetic sealing is not required (although as seen below in connection with the description of FIGS. 8-15A may be used in some instances in order to keep costs low) because there are no connections or internal components to fail.

One disadvantage of passive coil transmission as is used with the present invention is that the coupling efficiency, and thus the level of the signal presented to the electrode, is highly dependent on the spacing between the transmitting and receiving coils, i.e., the spacing between the external coils 54 included in the headpiece 22 and the coil assemblies 28 included within the implantable array 12. Such spacing is determined by the thickness of the skin 16 of the individual patient and in the alignment of the coils. The alignment of the coils is optimized as described above. The skin thickness will vary somewhat from patient to patient. It is recommended, therefore, that during implant of the array 12, the skin flap that is placed over the proximal end 30 of the implantable array 12 be thinned down to the bottom of the hair follicles. Such procedure should keep the skin thickness to a minimum. Suitable signal strength adjustments may then be made in the WP 18, after implant, to compensate for individual variations in skin thickness.

One of the key aspects of the invention is the use of the biphasic pulses in a fast CIS strategy. Such biphasic pulses have a frequency content that is sufficiently high to permit the pulses to be inductively coupled, through a layer of skin, from the external coil 54 to the implanted coil 28 with reasonable efficiency. As a result, there is no need in the invention to generate and modulate a high frequency carrier signal (to carry information from the external unit to the implanted unit) as has been required in prior art devices that employ inductive coupling as a data link, thereby further simplifying the invention and keeping its cost low. Such inductively-coupled link, through the use of two or more of such pairs of aligned coils (one external and one implanted), thus makes possible various multichannel embodiments of the invention.

Turning next to FIG. 5, a functional block diagram of the WP 18 and the headpiece 22 is shown. As seen in FIG. 5, the WP 18 includes a microphone 56. The microphone converts audible sounds, e.g., speech signals, into electrical signals as is known in the art. Such electrical signals are then amplified using a suitable preamplifier 60. The preamplified signal is then passed through a pre-emphasis filter 62. The pre-emphasis filter 62 is effectively a high-pass filter having a cut-off frequency of around 1200 Hz and a 6 dB/octave slope. Such slope causes the low frequency signals, i.e., those below the cut-off frequency, to be attenuated more than high frequency signals. The lower the frequency below the cut-off frequency, the more the attenuation. The net effect is to equalize the energy content of the amplified signal across the full spectrum because the incoming speech signal generally contains increased energy in the lower frequencies.

An automatic gain control (AGC) circuit 64, which may be of conventional design, then adjusts the overall signal level to compensate for differences in loudness of different speaker's voices or different distances of the speaker from the microphone 56. Such AGC circuit typically uses a rapid attack time of about 5 msec and a slow release time of around 200 msec to ensure that rapid transients will not overstimulate the listener and that soft sounds will not be lost. In combination, the microphone 56, preamplifier 60, pre-emphasis filter 62 and AGC circuit 64 may be functionally viewed as microphone means for generating an electrical signal representative of sensed audible sounds. Alternatively, for some applications, just a microphone and a preamplifier could be considered as a microphone means that generates electrical signals representative of sensed speech.

The full-band signal thus received and processed by the microphone means (e.g., the microphone 56, preamplifier 60, pre-emphasis filter 62 and AGC circuit 64) is then divided into four frequency bands by respective bandpass filters BP1, BP2, BP3 and BP4. In the preferred embodiment, the filter BP1 has a frequency band of from 100-700 Hz; BP2 has a frequency band of from 700-1400 Hz; BP3 has a frequency band of from 1400-2800 Hz, and BP4 has a frequency band of from 2800 Hz to 5600 Hz. These frequency bands correspond roughly to the natural acoustic tuning frequencies of the intact cochlea at the locations corresponding to the intended placement of the electrode contacts when the electrode array is inserted to its full intended depth within the cochlea.

Once divided into the four frequency bands or channels, appropriate speech processing means are used to process each of the plurality of frequency band signals in accordance with a prescribed speech processing strategy. Each band is processed in the same manner.

In the preferred embodiment, as shown in FIG. 5, the processing of the signal in each channel includes first subjecting the band-limited signal to a rectifier circuit, RECT1, RECT2, RECT3 or RECT4, followed by a low-pass filter LP1, LP2, LP3, or LP4. The combination of a rectifier circuit (which may be either a full or half wave rectifier circuit) followed by a low pass filter effectively derives the instantaneous envelope of the speech (or audio) signals in that band. The preferred low-pass filter cut-off frequency for the filters LP1-LP4 is about 200 Hz.

After deriving the instantaneous envelope signal of the speech signals in each channel, available at the output of each of the low-pass filters LP1, LP2, LP3 and LP4, the envelop waveform is then mapped from acoustic amplitudes to electrical amplitudes by a loudness mapping circuit LM1, LM2, LM3, or LM4. Loudness mapping is a critical element in the adjustment of the speech processor so that the loudness relations within and between speech sounds are preserved on each electrode. The preferred loudness mapping function is achieved by establishing a linear or logarithmic function between the acoustic and electrical amplitudes. The selected function is chosen to define, for each stimulation site, the amplitudes of the electrical stimulation that produce a signal that is at least equal to the threshold sensation, but not greater than the maximal comfortable loudness, over the anticipated dynamic range of the acoustic signal. The loudness mapping circuits LM1, LM2, LM3, and LM4 may thus be realized by a circuit that provides a transfer function that generates an output signal that is always in the range between threshold and maximal loudness. Such circuits are known in the art, and may range from a simple circuit that uses a P-N junction (diode or transistor) that is properly biased, to more complex circuits that involve numerous active filters and operational amplifiers, or DSP (digital signal processing) circuits that effectively provide a look-up table.

Once the signal has been appropriately mapped to convert the acoustical amplitude to an appropriate electrical signal amplitude, biphasic pulses having the determined electrical amplitude are then generated using a continuous interleaved sampling (CIS) technique. A biphasic pulse is one that has a short pulse of one polarity followed immediately (or almost immediately, e.g., within a few microseconds) by an opposite-polarity pulse of the same magnitude (width and amplitude). The CIS strategy stipulates that only one signal be sampled at any instant of time. However, the sampling rates may be very rapid, e.g, from 250 to 5000 Hz, with sampling pulse widths of from 10 to 250 μsec. To perform such sampling, a suitable oscillator/clock circuit 70 sets the basic sampling interval; and then an interleave pulse modulator 72 directs the sampled pulse to a gate or multiplier circuit 74-77 of one of the four channels.

It is noted that circuits that employ a CIS strategy as described above are known in the art. The present invention is not directed to a CIS strategy. Rather, it is directed to a low-cost multichannel cochlear implant device/system that employs a fast CIS strategy. The narrow pulses associated with such a fast CIS strategy advantageously have their energy at a sufficiently high frequency to be amendable to inductive coupling through a pair of aligned coils on either side of the patient's skin. With such inductive coupling, it is thus possible to use the coupled energy directly as a stimulation pulse, thereby allowing the implant device to be extremely simple consisting of just a coil having its ends connected through suitable wires to respective electrodes for each channel.

The above signal processing techniques, as well as the CIS processing strategy, are further described in connection with FIGS. 6 and 7. In FIG. 6, waveform A symbolically represents the incoming full-band signal that is sensed by the microphone 56 and processed by the preamplifier 60, pre-emphasis filter 62, and AGC circuit 64. The dotted line portion of waveforms "B", "C", "D" and "E" represent, for each channel, the instantaneous envelope waveforms obtained at the output the low pass filters LP1, LP2, LP3 and LP4, and as mapped using the loudness mapping circuits LM1, LM2, LM3 and LM4, respectively.

At a first instant of time, a first sample pulse 80 samples the "B" waveform (Channel 1) and determines its amplitude at the sample time. An appropriate biphasic sampling pulse of that amplitude is then generated. At the next sampling time, a second sample pulse 82 samples the "C" waveform (Channel 2) and determines its amplitude at the sample time, generating a corresponding biphasic sampling pulse. At the next sampling time, a third sample pulse 84 samples the "D" waveform (Channel 3) and determines its amplitude at the sample time, generating a corresponding biphasic sampling pulse. At the next sampling time, a fourth sample pulse 86 samples the "E" waveform (Channel 4) and determines its amplitude at the sample time, generating a corresponding biphasic sampling pulse. At the next sampling time, a fifth sample pulse 88 samples the "B" waveform (Channel 1) again, and the process continues in this manner, with one channel being sampled each sampling time.

It should be pointed out that that which is shown in FIG. 6 is not drawn to scale, particularly relative to the time (horizontal) axis. In the CIS strategy employed, the sampling occurs at a very rapid rate relative to the signal rates in the waveform(s) that is being sampled. Hence, if FIG. 6 were drawn appropriately, the sampling pulses would be much narrower than shown, and much closer together.

FIG. 7 shows the results, on an expanded scale, of applying a CIS strategy to a 4 channel system. A biphasic pulse 90 has an amplitude (both positive and negative) that is a function of the processed waveform (i.e., the band-limited signal that has been rectified, filtered, and mapped for loudness) at the time of sampling. The interchannel sampling time is "T". Thus, T seconds after the pulse 90 is generated, another biphasic pulse 92 is generated that has an amplitude determined by the sampled Channel 2 waveform. In a similar manner, biphasic pulses 94 and 96 are generated as a function of the amplitude of the sampled Channel 3 and Channel 4 waveforms, respectively. The process then repeats, with another biphasic pulse being generated for Channel 1 T seconds after the pulse 96, or 4 T seconds after the last pulse for Channel 1.

Returning momentarily to FIG. 5, the biphasic pulses generated by the interleave pulse modulator 72, in combination with the multiplier/gate circuits 74-77, are applied to the coils 54 of the headpiece 22 via the cable 58. Preferably, a gain control device G1, G2, G3, and G4, is inserted in each line so that the overall amplitude of the biphasic stimulation pulses applied to that channel can be adjusted by the user, or an audiologist, to compensate for variations in skin thickness, etc. Such gain control device may be as simple as a potentiometer that attenuates the biphasic pulse by an amount controlled by a manual setting. More sophisticated gain control devices may also be used, such as active amplifier/driver circuits that include gain adjustment features.

When the biphasic pulse is applied to the appropriate coil 54 of the headpiece, it is coupled to the implanted coil 28 of the implantable array 12. Such coupling tends to smooth the waveform, as shown in the bottom portion of FIG. 7. Thus, the current resulting from application of the pulse 90 to one of the headpiece coils 54 causes a biphasic current pulse 100 to flow between the electrodes of the implantable array 12 connected to the particular coil 28 that is coupled to receive the pulse 90. In a similar fashion, biphasic current pulses 102-108 flow between appropriate electrodes of the implantable array 12 corresponding to application of the biphasic pulses 92-98, respectively, to the headpiece coils 54. Note that in every case the current at the electrodes is biphasic, and that only one current is applied to any pair of electrodes at any instant of time.

Hermetic Sealing vs. Non-hermetic Sealing

As indicated above, one embodiment of the invention contemplates that all materials used within the implantable array 12 be made from biocompatible materials so that the array 12 need not be hermetically sealed. For example, pure platinum wire could be used as the wire to wind all of the receiving coils due to its biocompatibility and non-corrosive property. However, at current market prices, an all-platinum design appears to be prohibitive due to the expense of the raw platinum materials. While other biocompatible materials could be used, such as stainless steel and/or tantalum, use of such materials potentially creates a different set of problems. For example, stainless steel is a relatively poor electrical conductor, and tantalum requires a suitable insulation layer, which insulation layer using current insulation materials, such as Teflon, is too thick (particularly for the wound coils).

Without ruling out the possibility of a non-hermetically-sealed implantable array 12, another alternative is simply to house the receiver portion (the four receiving coils) of the implantable array within an hermetically-sealed case. Advantageously, hermetic sealing is a proven technology that has been used in pacemaker and other implant technologies for many years. Further, because the cost of hermetic sealing has dramatically decreased in recent years, it may actually prove significantly less expensive to place both the four-coil headpiece and four-coil implantable receiver in matching hermetically sealed cases. Such a system--using an hermetically-sealed four-coil receiver case--is next described in connection with FIGS. 8 through 15B.

A key aspect of the hermetically-sealed four-coil interface to be described next is arriving at an accurate model of the coil-interface so that verification of the desired system specifications can be confirmed using a given coil design. The desired specifications for the coil interface design stipulate that the coil interface should have: (1) no active internal components and a biocompatible encapsulation; (2) a resonant frequency near 10 kHz to allow effective transmission of biphasic pulses with pulse durations on the order of 50 μs (microseconds) per phase; (3) a gain of at least -2 dB at the resoant frequency when there is a maximum 9 mm separation between the external and internal coils (including skin thickness and coil encapsulation); (4) no significant ringing between two adjacent pulses; (5) less than 10% cross-talk between two adjacent coil pairs; (6) minimal size (<40 mm in diameter and <5 mm in thickness); (7) reasonable power efficiency so as to provide a battery life w/in the external WP of at least 10 hours continuous stimulation; and (8) an easy and reliable alignment method between the external and internal coils.

The approach used to arrive at a model of the coil interface is summarized in FIG. 8. Advantageously, the model takes into account both geometrical and material parameters in order to derive a frequency response of the coil transmission system. Once the model is developed, then a design optimization procedure may be used to optimize the coil design as also depicted in FIG. 8. The model is developed, as seen in FIG. 8 by arriving at a first model validation (block 120 in FIG. 8). Such first validation is based on a prescribed type of coil interface with known materials, e.g., copper, and a specified number of turns. An electric circuit model based on the prescribed interface and known materials is then developed (block 122) with the aid of published geometrical data. See, e.g., Grover, F. W., Inductance Calculations: Working formulas and tables, (D. Van Nostrand Co., New York, 1946). Prototypes of the proposed coil designs may then be made (block 126) and then a second validation is performed (block 128) to see how well the prototype coils perform against the model. Adjustments are made as required to the electric circuit model and geometrical model in order to achieve validation of the model.

In this instance, the second validation (block 128) was achieved by measuring the data from three different types of prototype coils and comparing such measured data to published data, e.g., the empirical data published by F. W. Grover (1946), supra. An example of such comparison is depicted in the frequency response graphs shown in FIGS. 9A, 9B and 9C.

With the coil model validated (at block 128), the specifications and constraints of the coil interface are then invoked (block 120) so that the remaining coil interface parameters, i.e., those relating to space variables, may be determined (block 132). These space variables include, the outside diameter of the coil "A", the thickness of the coil "b", the width of the wire portion of the coil "c" (i.e., the difference between an outer radius and inner radius of the coil), the size or gage of the wire (AWG); and the spacing "ρ" between the coils. The "A", "b" and "c" parameters are further illustrated and defined in FIGS. 10A and 10B.

The space variables are then varied, using the coil models, in an interactive fashion, until the specification is matched (block 134). Once a match of the desired specification has been achieved, such match yields a desired coil interface design (block 136), or a "solution" to the coil interface problem being solved.

In this instance, over 2 million possible coil coupling combinations were analyzed, resulting in a final coil interface design, or solution, that satisfies requirements of both transmission efficiency and power consumption. The design parameters of such solution are summarized below in Table 1.

                  TABLE 1                                                          ______________________________________                                         COIL INTERFACE DESIGN PARAMETERS                                               ______________________________________                                         Coil 1 (External/Transmitter Coil):                                            A1 = 1.4 cm                                                                    b1 = 0.22 cm                                                                   c1 = 0.35 cm                                                                   AWG = 31.00                                                                    N1 (turns) = 123                                                               Material = copper                                                              Coil 2 (Implanted/Receiving Coil):                                             A2 = 1.40 cm                                                                   b2 = 0.22 cm                                                                   c2 = 0.35 cm                                                                   AWG = 42.00 r                                                                  N2 (turns) = 1706                                                              Material = copper                                                              Intercoil Parameters:                                                          Center Frequency = 10.63 kHz                                                   3dB low side band = 1.87 KHZ                                                   3dB high side band = 60.36 KHZ                                                 Magnitude of center frequency = -1.31 dB                                       Frequency Bandwidth = 58.49 KHz                                                Separation between Coil 1 and Coil 2 = 9 mm                                    ______________________________________                                    

Turning next to FIGS. 11A and 11B, a planar and side (or edge) view of a preferred implanted receiver coil 140 is shown. The case 140 includes four coils 142, each having general design parameters as specified in Table 1 for Coil 2. In the preferred configuration, the diameter of the case is about 3.7 cm and its thickness is about 3.8 mm. Five feedthroughs 146 allow electrical connection with the coils 142. The feedthroughs 146 may be made from, e.g., 100% platinum, or other suitable biocompatible material. Four of the feedthroughs are connected to one end of each of the coils 142. One of the feedthroughs is connected to the other end of all of the coils 142 so as to form a common ground connection. The case 140 is round and is made from a biocompatible material, e.g., titanium, and is laser welded so as to form an hermetic seal. A ceramic or glass bridge 148 holds and maintains the feedthroughs 146 in a spaced-apart relationship. Hermetic bonding between the ceramic or glass bridge 148 and the titanium case 140 may be realized using bonding techniques known in the art. See, e.g., U.S. Pat. Nos. 4,991,582 or 5,513,793, both of which patents are incorporated herein by reference. The internal parts used within the case 140 are entirely passive and are biocompatible with the hermetically-sealed package. The feedthroughs 146 are electrically and mechanically connected to respective biocompatible wires 40 embedded or enclosed within the carrier 26 (see FIGS. 1 and 2), each of which is connected to an electrode at the distal end of the carrier, as previously explained. Such connection to the feedthroughs 146 may be made directly, e.g., by permanently bonding a respective wire 40 to a corresponding feedthrough 146 exposed on the outside of the case 140, and then encasing the exposed feedthroughs and wires within the carrier 26; or such connection may be made through the use of a suitable connector adapted to mate with the exposed feedthroughs 146.

Turning next to FIG. 12, the measured frequency response of an implanted four-coil receiver assembly made as shown in FIG. 11A is compared to the frequency response predicted by the model both with (filled triangles) and without (open circles) the titanium case 140. It is noteworthy that the frequency response of the model (solid line) closely tracks that of the measured response. Further, and perhaps more significantly for purposes of using the invention, the presence of the titanium case does not significantly decrease the transmission efficiency at the frequency of interest (i.e., about 10 KHz, for 50 μs per phase biphasic pulses). Above 10 KHZ, the titanium case does decrease the transmission efficiency by a noticeable amount.

FIG. 13 shows the actual biphasic waveforms measured at the input and output of a four-coil transmitter/receiver assembly made to the specifications of Table 1. The top waveform in FIG. 13 is the input biphasic pulse train applied to the transmitting coil (Coil 1 in Table 1). The bottom waveform in FIG. 13 is the output biphasic pulse train appearing at the receiving coil (Coil 2 in Table 1). The vertical scale for both the top and bottom waveforms in FIG. 13 is 1.0 volts/division. The horizontal scale is 200 μs per division. The output pulse is rounded slightly due to the limited bandwidth of the coil transmission. Significantly, there is no noticeable ringing between the two pulses, which is one of the desired performance specifications for the assembly.

Turning next to FIG. 14, the measured cross-talk between coils is graphically displayed. A direct transmission was applied to a first coil (filled squares connected by a solid line), and the measured cross-talk at the other three coils (dashed lines) as a function of coil separation (skin thickness) is shown. By way of example, and with reference to FIG. 14, cross-talk "1-2" indicates the input signal is applied to the input of external (transmitting) Coil #1, and the cross-talk signal at the output of internal/implanted Coil #2 is measured. Significantly, as seen by the data in FIG. 14, the cross-talk signal measured at Coil #2 is at least 20 dB lower than the direct transmission over essentially all possible ranges of skin thickness (coil separation).

As previously indicated in reference to FIGS. 1 and 2, some type of means 50 and 52 must be employed to assure proper alignment between the external transmitting coils and the implanted receiving coils. A preferred magnetic alignment means is illustrated in FIGS. 15A and 15B. FIG. 15A shows a planar representation of the implanted coil assembly 141 as in FIG. 11A, with a first cylindrical magnet 148 placed inside of a first coil 142' so that its South pole is "up", and with a second cylindrical magnet 150 placed inside of a second coil 142" so that its North pole is "up". FIG. 15B shows a similar representation of an external coil assembly 152 having four coils 154 therein (made, e.g., to the specifications indicated in Table 1 for Coil 1). A first cylindrical magnet 156 is placed inside of a first external coil 154' so that its North pole is "up"; and a second cylindrical magnet 158 is placed inside of a second external coil 154" so that its South pole is "up". With such configuration, there is only one correct way in which the implanted coil assembly 151 can align with the external coil assembly 152: namely, the south pole of implanted magnet 148 aligns with the north pole of external magnet 156 (thereby aligning the external coil 154' with internal coil 142'), and the south pole of external magnet 158 aligns with the north pole of implanted magnet 150 (thereby aligning external coil 154" with internal coil 142"). Note, for purposes of FIGS. 15A and 15B, a filled circle for a magnet signifies a north pole, while a cross-hatched circle for a magnet signifies a south pole.

As described above, it is thus seen that a passive, hermetically-sealed transmission interface may be used with a four-channel (CIS) speech processor and a four-electrode monopolar intracochlear electrode array to provide a relatively inexpensive, cochlear stimulator system.

As further described above, it is seen that the invention, whether employing an hermetically-sealed or non hermetically-sealed transmission interface, provides a low-cost, four-channel cochlear stimulation system. The system utilizes a completely passive, implantable electrode array having four electrodes, four receiving coils, and a reference electrode. The coils and electrodes may be made from inexpensive metals, and carried in a carrier of silicon rubber. One end of the carrier may be pre-formed in a spiral to match the basal turn of the cochlea, with the four electrodes being spaced apart along the inner radius of the spiral.

It is further seen that the invention provides an inexpensive cochlear stimulation system that offers the best combination of reliability, low cost and robust medical properties for widespread application. The invention thus has widespread application in underdeveloped areas, requiring minimal medical supervision and attention once implanted.

It is also seen that the invention provides an inexpensive cochlear stimulation system that utilizes the frequency content of brief biphasic pulses generated in multiple channels of a CIS processing strategy to permit such pulses to be inductively coupled through the skin of a patient to an aligned implanted coil with reasonable efficiency. Such pulses are then used directly as a stimulating signal at electrodes connected to the implanted coil without the need for intervening implanted electrical or electronic circuitry.

While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. 

What is claimed is:
 1. A low-cost, multi-channel cochlear stimulation system comprising:a passive, implantable receiver/electrode array comprising:an elongate silicone rubber carrier in which is embedded a plurality of coil assemblies at a proximal end of the carrier and a corresponding plurality of electrodes at a distal end of the carrier, the distal end of the carrier being adapted for insertion into a human cochlea, with the plurality of electrodes being spaced apart along the distal end, the proximal end of the carrier holding the plurality of coil assemblies in a predetermined pattern, the coil assemblies each comprising a prescribed number of turns of a first wire wound to form a coil, a first end of the first wire of each coil assembly being bonded to a second wire that extends the length of the elongate carrier inside of the carrier to a respective electrode at the distal end of the carrier, second end of the first wire of each coil assembly being bonded to a third wire that extends less than the length of the elongate carrier to a common electrode exposed at the surface of the carrier, each electrode at the distal end of the carrier comprising a ball formed by flaming a distal end of the second wire, a portion of which ball is exposed through the surface of the carrier, implantable alignment means for aligning the plurality of coil assemblies with corresponding external coils, and an hermetically sealed case made from a biocompatible material that houses said coil assemblies; and an external wearable processor comprising:microphone means for generating an electrical signal representative of sensed audible sounds, filter means for dividing the electrical signal into a plurality of frequency bands, thereby creating a corresponding plurality of frequency band signals, the number of frequency bands being the same as the number of implanted coil assemblies and electrodes, whereby there is an implanted coil assembly and corresponding electrode for each filter means, speech processing means for processing each of the plurality of frequency band signals in accordance with a prescribed speech processing strategy to produce respective biphasic stimulation signals, and an external headpiece having a plurality of external coils, each connected to receive one of the respective biphasic stimulation signals, external alignment means for positioning the external headpiece in alignment with the implantable alignment means to align each of the external coils of the external headpiece with respective ones of the plurality of coil assemblies of the implantable receiver/electrode array; whereby the biphasic stimulation signals received by each of the plurality of external coils are inductively coupled to respective ones of the plurality of implantable coil assemblies, thereby causing a biphasic stimulation current to flow between respective ones of the implantable electrodes and the common electrode.
 2. The cochlear stimulation system of claim 1 wherein the biocompatible material from which the hermetically sealed case is made comprises titanium.
 3. The cochlear stimulation system of claim 1 wherein the hermetically sealed case has four identical coil assemblies housed therewithin, each coil assembly having the general shape of a doughnut with an opening therein.
 4. The cochlear stimulation system of claim 3 wherein the implantable alignment means comprises first and second internal permanent magnets positioned within the opening of corresponding first and second coil assemblies, with the orientation of the first internal permanent magnet within its respective opening establishing a magnetic polarity for the first internal permanent magnet that is opposite to the magnetic polarity established by the second internal permanent magnet as a result of the second internal permanent magnet's orientation within its respective opening.
 5. The cochlear stimulation system of claim 4 wherein the external headpiece comprises a sealed external case having a size and shape substantially the same as the hermetically sealed case, and wherein each of the four external coils has a size and a shape substantially the same as the size and shape of the four coil assemblies housed within the hermetically-sealed case, and further wherein the external alignment means comprises first and second external magnets positioned within corresponding first and second external coils, the magnetic polarity of the first and second external magnets being such that only one correct orientation is possible between the external case and the hermetically-sealed case when the magnetic polarities of the first and second external magnets align with opposite magnetic polarities of the first and second internal permanent magnets.
 6. The cochlear stimulation system of claim 5 wherein the hermetically-sealed case of the implantable receiver/electrode array has a diameter of no greater than about 3.7 cm, and a thickness of no greater than about 3.8 mm.
 7. The cochlear stimulation system of claim 5 wherein each of the external coils in the external case comprises at least 120 turns of gage AWG 31.00 copper wire, and wherein each of the coil assemblies within the hermetically-sealed case comprises a coil of at least 1700 turns of gage AWG 42.00 copper wire.
 8. The cochlear stimulation system of claim 7 wherein each of the external coils in the external case and each of the coils within the hermetically-sealed case have a diameter of no greater than about 1.40 cm, and a thickness of no greater than about 0.22 cm.
 9. The cochlear stimulation system of claim 1 wherein the plurality of coil assemblies and corresponding electrodes of the implantable receiver/electrode array comprises four coil assemblies and four corresponding electrodes, whereby the cochlear stimulation system comprises a four channel cochlear stimulation system.
 10. The cochlear stimulation system of claim 9 wherein the plurality of frequency bands into which the filter means divides the electrical signal comprises four frequency bands, a first band from 350 Hz to 700 Hz, a second band from 700 Hz to 1400 Hz, a third band from 1400 Hz to 2800 Hz, and a fourth band above 2800 Hz.
 11. The cochlear stimulation system of claim 10 wherein the microphone means comprisesa microphone, a preamplifier coupled to the microphone, pre-emphasis filter means for equalizing the energy content of an output signal received from the preamplifier, and automatic gain control means for adjusting an output signal from the pre-emphasis filter means to compensate for differences in loudness of different audible sounds sensed by the microphone.
 12. The cochlear stimulation system of claim 11 wherein the speech processing means includes first, second, third and fourth rectifier circuits for rectifying the corresponding frequency band signals; and first, second, third and fourth low pass filter means connected to receive an output of the first, second, third, and fourth rectifier circuits, respectively, for deriving an instantaneous envelope signal of the content of the respective frequency band signals; and loudness mapping means for mapping the instantaneous envelope signal into electrical amplitudes for the respective biphasic stimulation signals.
 13. The cochlear stimulation system of claim 12 wherein the speech processing means further includes means for sampling the instantaneous envelope signal of the respective frequency band signals, and mapping the sampled signal to an appropriate biphasic stimulation amplitude, on a continuous interleaved sampled (CIS) basis, with only one envelope signal being sampled, and hence with only one biphasic stimulation signal being generated, at any instant of time.
 14. The cochlear stimulation system of claim 12 wherein loudness mapping means applies a logarithmic function between the instantaneous envelope signal and the electrical amplitude of the resulting biphasic stimulation signal.
 15. A passive, implantable receiver/electrode array for use with a four-channel cochlear stimulation system consisting of:an elongate silicone rubber carrier in which is embedded four coil assemblies at a proximal end of the carrier and four electrodes at a distal end of the carrier, the distal end of the carrier being formed in the shape of a spiral to match the basal turn of a human cochlea, with the four electrodes being spaced apart along an inner radius of the spiral, the proximal end of the carrier holding the four coil assemblies in a predetermined pattern, the coil assemblies each comprising a prescribed number of turns of a first wire wound to form a coil, a first end of the first wire of each coil assembly being bonded to a second wire that extends the length of the elongate carrier inside of the carrier to a respective electrode at the distal end of the carrier, a second end of the first wire of each coil assembly being bonded to a third wire that extends less than the length of the elongate carrier to a common electrode exposed at the surface of the carrier, each electrode at the distal end of the carrier comprising a ball formed by flaming a distal end of the second wire, a portion of which ball is exposed through the surface of the carrier, implantable alignment means for aligning the four coil assemblies with corresponding external coils, and an hermetically sealed case made from a biocompatible material that houses said four coil assemblies.
 16. The cochlear stimulation system of claim 15 wherein the biocompatible material from which the hermetically sealed case is made comprises titanium.
 17. The cochlear stimulation system of claim 15 wherein the hermetically sealed case has four identical coil assemblies housed therewithin, each coil assembly having the general shape of a doughnut with an opening therein.
 18. The cochlear stimulation system of claim 17 wherein the implantable alignment means comprises first and second internal permanent magnets positioned within the opening of corresponding first and second coil assemblies, with the orientation of the first internal permanent magnet establishing a magnetic polarity for the first internal permanent magnet that is opposite to the magnetic polarity established by the second internal permanent magnet as a result of the second internal permanent magnet's orientation.
 19. A method of optimizing a coil interface used within a low cost, multichannel cochlear implant system, said system having a plurality of implantable receiver coils, and a corresponding plurality of external transmitting coils, the implantable receiving coils being housed within an hermetically-sealed case, the method comprising the steps of:(a) developing an electric circuit model and a geometrical model of the coil interface based on empirical data; (b) fabricating a prototype coil interface using the models developed in step (a); (c) validating the prototype coil interface by adjusting the electric circuit model and geometrical model as required to meet desired performance criteria; (d) applying desired interface specifications and constraints to the validated prototype coil interface of step (c); (e) adjusting and configuring space variables associated with the coil interface model in an iterative fashion until the performance of the interface model matches the interface specifications and constraints applied in step (d), said space variables including coil diameter, coil thickness, coil width, wire size and coil spacing; and (f) using the space variables arrived at in step (e) as the design parameters for an optimized coil interface.
 20. A passive, implantable receiver/electrode array for use with a multichannel cochlear stimulation system consisting of:a flexible carrier in which is embedded a plurality of coil assemblies at a proximal end of the carrier and a corresponding plurality of electrodes at a distal end of the carrier, the distal end of the carrier being adapted for insertion into a human cochlea, with the plurality of electrodes being spaced apart along the distal end, the proximal end of the carrier holding the plurality of coil assemblies in a predetermined pattern, the coil assemblies each comprising a prescribed number of turns of a first wire wound to form a coil, a first end of the first wire of each coil assembly being electrically coupled to a second wire that extends the length of the carrier to a respective electrode at the distal end of the carrier, a second end of the first wire of each coil assembly being electrically coupled to a third wire that extends less than the length of the carrier to a common electrode exposed at the surface of the carrier, each electrode at the distal end of the carrier comprising a smoothed surface formed by flaming a distal end of the second wire, a portion of which smoothed surface is exposed through the surface of the carrier, implantable alignment means for aligning the plurality of coil assemblies with corresponding external coils, and an hermetically sealed case made from a biocompatible material that houses said plurality of coil assemblies. 